This invention relates to a thick-film, inorganic, electroluminescent (EL) panel and, in particular, to an EL panel assembled on a release layer and, after separation from the release layer, an EL panel that does not substantially curl or distort.
As used herein, and as understood by those of skill in the art, “thick-film” refers to one type of EL lamp and “thin-film” refers to another type of EL lamp. The terms only broadly relate to actual thickness and actually identify distinct disciplines. In general, thin film EL lamps are made by vacuum deposition of the various layers, usually on a glass substrate or on a preceding layer. Thick-film EL lamps are generally made by depositing layers of inks on a substrate, e.g. by roll coating, spraying, or various printing techniques. The techniques for depositing ink are not exclusive, although the several lamp layers are typically deposited in the same manner, e.g. by screen printing. A thin, thick-film EL lamp is not a contradiction in terms and such a lamp is considerably thicker than a thin film EL lamp.
In the context of a thick-film EL lamp, and as understood by those of skill in the art, “inorganic” refers to a crystalline, luminescent material that does not contain silicon or gallium. The term does not refer to the other materials from which an EL lamp is made.
As used herein, an EL “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.” An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current.
EL phosphor particles are typically zinc sulfide-based materials, including one or more compounds such as copper sulfide (Cu2S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors typically contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. The color of the emitted light is determined by the doping levels. Although understood in principle, the luminance of an EL phosphor particle is not understood in detail. The luminance of the phosphor degrades with time and usage, more so if the phosphor is exposed to moisture or high frequency (greater than 1,000 hertz) alternating current.
Various colors can be produced by mixing phosphors having different dopants or by “color cascading” phosphors. A copper-activated zinc sulfide phosphor produces blue and green light under an applied electric field and a copper/manganese-activated zinc sulfide produces orange light under an applied electric field. Together, the phosphors produce what appears to be white light. It has long been known in the art to color-cascade phosphors, i.e. to use the light emitted by one phosphor to stimulate another phosphor or other material to emit light at a longer wavelength; e.g. see U.S. Pat. No. 3,052,810 (Mash). It is also known to doubly cascade light-emitting materials. U.S. Pat. No. 6,023,371 (Onitsuka et al.) discloses an EL lamp that emits blue light coated with a layer containing fluorescent dye and fluorescent pigment. In one example, the pigment absorbs blue light and emits green light, while the dye absorbs green light and emits red light.
A modern (post-1985) EL lamp typically includes transparent substrate of polyester or polycarbonate material having a thickness of about 7.0 mils (0.178 mm.). A transparent, front electrode of indium tin oxide or indium oxide is vacuum deposited onto the substrate to a thickness of 1000 Å or so. A phosphor layer is screen printed over the front electrode and a dielectric layer is screen printed over phosphor layer. A rear electrode is screen printed over the dielectric layer. It is also known in the art to deposit the layers by roll coating.
The inks used include a binder, a solvent, and a filler, wherein the filler determines the nature of the ink. A typical solvent is dimethylacetamide (DMAC). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, epoxy, or Kynar 9301, a proprietary terpolymer sold by Atofina. A phosphor layer is typically screen printed from a slurry containing a solvent, a binder, and zinc sulphide particles. A dielectric layer is typically screen printed from a slurry containing a solvent, a binder, and particles of titania (TiO2) or barium titanate (BaTiO3). A rear (opaque) electrode is typically screen printed from a slurry containing a solvent, a binder, and conductive particles such as silver or carbon.
As long known in the art, having the solvent and binder for each layer be chemically the same or chemically similar provides chemical compatibility and good adhesion between adjacent layers; e.g., see U.S. Pat. No. 4,816,717 (Harper et al.). It is not easy to find chemically compatible phosphors, dyes, binders, fillers, solvents or carriers and to produce, after curing, the desired physical properties, such as flexibility, and the desired optical properties, such as color and brightness.
An EL lamp constructed in accordance with the prior art is relatively stiff, even though it is typically only seven mils thick, making the lamp unsuited to some applications requiring greater flexibility, such as keypads. Layer thickness and stiffness are not directly related. The material from which the layer is made affects stiffness. Typically, EL lamps are made from the materials listed above. An EL lamp backlighting a keypad, for example, typically has holes under the keys to avoid affecting the actuation of a key. Simply reducing the thickness of the substrate does not provide the desired flexibility.
Relatively flexible EL lamps are known in the art. U.S. Pat. No. 5,856,030 (Burrows) discloses an EL lamp made on a UV-cured urethane layer on a release paper. The release paper provides substantial structural support while the lamp layers are applied from an ink containing a vinyl gel. Unlike panels made on substrates that are seven mils thick, or so, EL panels made on thin sheets from flexible materials, e.g. urethane one to five mils thick, do not keep their shape but bend or curl. This makes it extremely difficult to automate the assembly of panels into end products, e.g. a keypad for a cellular telephone or as the luminous structure in a three dimensional molded object.
Published PCT application WO 02/103718 alludes to “selected” layers of an EL structure being made from UV-curable inks. No basis for selection is described nor is any layer described that is not made from a UV-curable ink. U.S. Pat. No. 5,565,733 (Krafcik et al.) discloses an EL lamp made from solvent based materials and including a UV-curable dielectric layer overlying portions of conductive traces that are not connection points for the EL lamp. U.S. Pat. No. 5,770,920 (Eckersley et al.) discloses a UV-curable insulating layer overlying the rear electrode of an EL lamp made with solvent based materials. U.S. Pat. No. 5,780,965 (Cass et al.) discloses a polyurethane acrylic protective layer for an EL lamp. In general, the industry has followed the layers-having-similar-chemistry maxim pronounced in the Harper et al. patent, particularly for the lamp layers (between and including the electrodes).
In view of the foregoing, it is therefore an object of the invention to provide a thin, thick-film, inorganic EL panel that does not curl or distort when removed from a release layer.
Another object of the invention to provide a flexible EL lamp that is more stable dimensionally than urethane based EL lamps of the prior art.
A further object of the invention is to provide a flexible EL lamp that does not require similar chemistry for adjacent lamp layers.
Another object of the invention is to provide an EL lamp made from solvent based inks on a removable substrate or release layer.
A further object of the invention is to provide a flexible EL lamp that is brighter than flexible EL lamps of the prior art.
Another object of the invention is to provide a flexible EL lamp suitable for keypads.